1. Field of the Disclosure
The disclosure relates to an amplifier. More particularly, the disclosure relates to a current-mode amplifier.
2. Description of Related Art
In an ultra-wideband (UWB) wireless transceiver system, signal data is transmitted by orthogonal frequency division multiplexing (OFDM). To decode the signal data at a transceiver end, during a process of reducing signal from a radio frequency (RF) to a baseband (0-240 MHz), and an operation of an analog to digital converter (ADC), a gain of the baseband must be consistent. However, in a present circuit design, a voltage-mode amplifier cannot be operated over 100 MHz. In a receiver of the UWB system, since the circuit is usually operated in a non-linear zone due to interference signals during a process of reducing the signal from the RF to the baseband, a linearity consideration is very important. In the UWB system, a variable gain amplifier must have a good linearity, an optimal direct current (DC) offset and an acceptable anti-noise function.
In a conventional IF down-conversion design, design difficulties and power consumptions of the variable gain amplifier and a filter are increased as a bandwidth thereof is increased. Also the IF down-conversion can resolve the problem of DC offset, other problems are encountered, for example, linearity and power consumption. Therefore, a direction-converter is still widely used in the UWB system. Anyway, in the present UWB system, the voltage-mode amplifier is used to implement the variable gain amplifier. An input impedance of the voltage-mode amplifier is the greater the better (which preferably approaches infinity), and an output impedance thereof is the smaller the better (which preferably approaches 0). Contrary to the voltage-mode amplifier, an input impedance of a current-mode amplifier is the smaller the better (which preferably approaches 0), and an output impedance thereof is the greater the better (which preferably approaches infinity). In the present UWB system, the current-mode amplifier is not yet used to implement the variable gain amplifier.